Orbit
Manufacturers of tandems.
The Libra design couild be useful with a child on the back.

Southwater Cycles
Not specialist tandem dealers, but they do have tandems for hire, so if you are not sure give them a go and learn how to ride in the peace and quite of the Downs Link cycle path.

Mercian Cycles
Good reputation, slightly old fashioned designs.
Based in Derbyshire

George Longstaff
For many years the real standard setter for great riding well built tandems.

Landescape Tandems
Interesting tandems with the option of two sets of wheels, 700C and 26" MTB so you can have fast wheels for those flat rides or touring, strong 26" wheels for off-road and expeditions.

St John Street Cycles
Based in Bridgwater, Somerset. Producer of Thorn tandems. Extensive range from budget to top spec, including their Raven version with Rohloff hub gear.
Also stock many hard to find items.

Roberts Cycles
Builder of highly desirable tandems and solos.
Very high quality fillet brazed joins.
Don't bel;ieve the delivery times and make sure they write down (and you keep a copy) of your specifications - they lost ours!